This invention relates to a control system for stopping a spindle at a predetermined position, and more particularly to a control system which is adapted to stop a spindle firmly at a predetermined rotational position with a high accuracy.
Machine tools with automatic tool changers are well known in the art. These are machine tools that perform machining work automatically while various tools are also changed automatically. The tools carried on such machines are changed in the following manner. First, a magazine holding a number of tools is revolved to bring a vacant tool holding portion of the magazine into position directly above a spindle mechanism. The spindle mechanism, which is grasping an old tool to be exchanged for a new one, is then projected forward, after which the magazine positioned above the spindle mechanism is lowered to permit the old tool to be received and grasped by the vacant tool holding portion of the magazine. The spindle mechanism is then retracted so that the old tool separates from the spindle, thus transferring the old tool to the magazine. Next, the magazine is revolved to bring a desired new tool into position in front of the spindle, and the spindle mechanism is projected forward to receive and grasp the new tool. Finally the magazine is raised away from the spindle to complete the tool change operation.
It is required in the tool change mechanism of the foregoing type that a prescribed part of the spindle, such as a key member, be stopped accurately at the correct rotational position in order to permit the fitting portions of the spindle and tool to mate smoothly with each other. More specifically, a key is mounted on the spindle and a key way is formed in the tool in order to mate with the key. The smooth mating of the spindle and tool requires that the spindle be positioned and stopped to provide the correct alignment of key and key way. Meeting the above requirement necessitates a high spindle positioning accuracy of from .+-.0.1.degree. to .+-.0.2.degree. in terms of the angle of rotation of the spindle.
The conventional automatic tool change mechanisms are provided with photoelectric detectors or with limit switch mechanisms which detect the rotational position of the spindle key in order to facilitate the smooth mating of the spindle and tools. The arrangement is such that the spindle is brought to a stop at the prescribed position by the application of a mechanical brake which is actuated in response to a signal from the key position detecting means.
The foregoing apparatus not only fails to provide the desired positioning accuracy but has a stopping mechanism that experiences wear with long use since the mechanism relies upon mechanical pin or brake means or the like. Such wear, particularly of a brake shoe or pin, makes it progressively more difficult to stop the spindle at the prescribed position as time passes, and the result is that the automatic exchanging of tools cannot proceed smoothly. Avoiding this situation entails troublesome maintenance and inspection work.
Stopping the spindle accurately at the predetermined rotational position is important not only when changing tools but also when withdrawing a tool from a workpiece in a boring operation. It is also important when attaching a square workpiece to, or detaching it from, a spindle in a lathe or the like. It is important in the former case in order to avoid flawing the workpiece, and in the latter case because the mounting and demounting of the workpiece would be impossible without correct positioning of the spindle. As pointed out above, however, stopping the spindle accurately at the correct position is not possible with the conventional apparatus, so that operability and workability decline.